Design requirements for internally cooled airfoils necessitate a positive pressure differential between the internal cooling air and the external hot gas environment to prevent hot gas intrusion into the airfoil in the event of an airfoil wall breach. CMC airfoils with hollow cores in gas turbines are particularly susceptible to wall bending loads associated with such pressure differentials due to the anisotropic strength behavior of CMC material. For laminate CMC constructions, the through-thickness direction has about 5% of the strength of the in-plane or fiber-direction strengths. Internal cooling air pressure causes high interlaminar tensile stresses in a hollow CMC airfoil, with maximum stress concentrations typically occurring at the inner radius of the trailing edge region. The inner radius of the leading edge region is also subject to stress concentrations.
This problem is accentuated in large airfoils with long chord length, such as those used in large land-based gas turbines. A longer internal chamber size results in increased bending moments on the walls of the airfoil, resulting in higher stresses for a given inner/outer pressure differential.
The most common method of reducing these stresses in metal turbine vanes is to provide internal metal spars that run the full or partial radial length of the airfoil. However this is not fully satisfactory for CMC airfoils, due to manufacturing constraints and also due to thermal radial expansion stress that builds between the hot airfoil skin and the cooler spars. Therefore, the present inventors have recognized that better methods are needed for reducing bending stresses in hot CMC airfoil walls resulting from internal cooling pressurization.